A halogen gas is used in an etching step in the fabrication of semiconductor devices. In order to prevent halogen gas corrosion of an etching apparatus, the inner side of the etching apparatus is generally protected by a thermal spray coating made of a highly anti-corrosive substance. Materials containing a rare earth element are often used as one type of such substances.
Known techniques relevant to thermal spray materials containing a rare earth element include a thermal spray material comprising a granulated powder material of a rare earth fluoride having an average primary particle size of 10 μm or smaller, an aspect ratio of 2 or smaller, an average particle size of 20 to 200 μm, and a degree of volume reduction of 30% or less (see Patent Literature 1). A spherically particulate thermal spray material formed of a compound containing a rare earth element including yttrium and having a fracture strength of 10 MPa or higher and an average particle size of 10 to 80 μm is also known (see Patent Literature 2).
The thermal spray material disclosed in Patent Literature 1 is produced by granulating a fluoride of a rare earth element together with a binder using a spray dryer, and firing the granules at a temperature of 600° C. or lower. Patent Literature 1 says in para. [0014] that if the firing temperature exceeds 600° C., an apparent weight loss results, indicating the occurrence of oxidative decomposition and, therefore, the firing for removing the binder should be carried out at or below 600° C. That is, Patent Literature 1 describes that the firing temperature should be 600° C. or lower so as not to cause the rare earth fluoride to oxidatively decompose to produce an oxyfluoride of the rare earth element. Granules of a rare earth fluoride (LnF3), such as those disclosed in Patent Literature 1, easily break due to cleavage fracture of LnF3 and therefore have the problem of instable feed to thermal spray equipment. Granules of LnF3 also have the problem that the coating film formed on a substrate by thermal spraying the granules is liable to separate from the substrate when subjected to a thermal shock.
The spherically particulate thermal spray material of Patent Literature 2 is produced by granulating a slurry of fine powder of a rare earth element-containing compound using a granulator and, when the compound is an oxide, firing the granules at 1200° to 1800° C. The Literature, however, gives no mention of firing conditions and the like for rare earth element-containing compound other than an oxide.